The present invention relates to an impact modifier composition which provides an improved balance of impact resistance and optical clarity to amorphous aromatic polyester resin (hereafter referred to as polyester). More specifically, the present invention concerns an impact modifier composition which contains a rubbery core, an intermediate hard stage, and a shell containing a hydroxyl group or another functional group which acts in a similar manner as the hydroxyl group.
Polyesters (e.g., polyethylene terephthalate, polybutylene terephthalate, etc.) exhibit various excellent properties such as resistance to temperature, chemicals, weathering, radiation and burning and also exhibit excellent clarity (in amorphous form), reasonable cost, as well as moldability. Accordingly, polyesters are used for various purposes (e.g., fibers, films, molded and extruded products, etc.). The impact resistance of the polyester, however, is less than satisfactory. Plastics such as poly (butylene terephthalate) (PBT) and poly(ethylene terephthalate) (PET) have insufficient impact strength, and many attempts have been made to improve the impact strength. Many agents have been proposed to improve the impact strength. These are added to resins and subjected to melt-blending.
Various attempts have been made using conventional fibrous inorganic fillers (e.g., glass fiber, asbestos fiber, etc.) to improve the impact resistance of polyester. Even when these methods are implemented, however, the impact resistance improvement is less than satisfactory and clarity in amorphous polyesters is adversely affected.
Various techniques wherein rubbery polymers or rubber- containing polymers are mixed with polyesters have been developed to improve the impact resistance of polyesters and thermoplastic materials. Specifically, certain core-shell polymers comprising a core made of rubbery polymer and a shell, around the core, made of a glassy polymer are excellent agents for improvement of impact strength of polyesters where clarity is not an object.
When these prior art methods are used, the polyester resin generally exhibits poor compatibility with the shell of the rubber-containing polymer, and therefore impact resistance is not fully optimized. Even when these prior art core-shell modifiers are added to amorphous polyesters and found to produce ductile, notched breaks, the clarity of amorphous polyester resins is destroyed. An amorphous polyester may contain a small amount of crystallinity, but the level must be low enough so that clarity is not adversely affected. Further, although the polyester may be crystallized under certain conditions, in the present invention the molding and cooling conditions are such that crystallization (and loss of clarity) is avoided.
Lane, U.S. Pat. No. 4,034,013 teaches core/shell polymers functionalized with an epoxy group, such as a shell of methyl methacrylate/glycidyl methacrylate, to improve the melt strength of polyesters. Although Lane broadly teaches butadiene-based elastomers with optional minor amounts of styrene in the core and teaches styrene as a major component of the outer stage, she does not teach or suggest a solution to preparing an efficient impact modifier which will retain clarity in the amorphous polyester.
Kishimoto et al., Japanese Kokai 54-48850, disclose butadiene-based core/shell polymers with hydroxyalkyl groups in the shell portion as modifiers for crystalline polyesters, such as poly(butylene terephthalate), but do not teach the means to modify such core/shell polymers to make them useful as impact modifiers in clear, amorphous polyesters.
Carson et al., U.S. Pat. No. 5,321,056 teaches impact modifiers which produce transparent, high notched Izod impact strength blends with amorphous aromatic polyesters. Carson's impact modifiers are core-shell polymers with cores comprised mainly of rubbery polymers of diolefins and vinyl aromatic monomers and shells comprised mainly of vinyl aromatic monomers and monomers containing a hydroxyl group. Although Carson provides for significantly improved impact strength of clear, amorphous aromatic polyester, a need still exists for a modifier that can provide an improved balance of properties between impact strength and optical clarity. Specifically, a need exists to reduce or eliminate the blue/yellow hue or tint that often results from blending such modifiers in amorphous aromatic polyester resins.
The object of the present invention is to provide a composition for improving the impact strength of polyesters, such as PET or PET copolyesters, when they are processed into clear, tough objects while retaining their amorphous nature. It is another objective that said composition provide improved impact properties without reducing the transparency of amorphous polyesters. A further objective is that the composition provide reduced blue/yellow hue, while maintaining desirable impact and transparency characteristics. Another object is to provide a composition which will also overcome the embrittlement caused by physical aging which commonly occurs in amorphous aromatic polyesters when conditioned at temperatures approaching glass transition temperature (Tg). A still further object is to provide clear amorphous extrusion/melt shaped or injection molded PET or PET copolyester articles.